To meet the increase in the volume of information and telecommunications in recent years, in the trunk line system of Japan, laying of a silica single mode fiber network and total digitalization have already been completed. The silica single mode optical fiber is an ideal transmission medium which is low in attenuation and which is capable of transmitting optical signals over a long distance. However, in order to make higher speed communication possible, the diameter of the core region is designed to be from about 5 to 10 μm. Further, there has been a practical problem such that it is extremely weak against bending due to inflexibility of silica, unless the diameter is made small.
In order to make it possible for optical signals to enter into, be connected to or be branched from such a small core region, a very high level of alignment technique is required. Accordingly, in order to construct all by the silica single mode fiber, an enormous cost will be required, and such construction has not yet been extended to a domestic household level.
On the other hand, a polymer optical fiber (hereinafter referred to simply as POF) is such that the base material constituting the fiber is made of a polymer, and thus the cost for the material is low, and as compared with the silica fiber, it is possible to enlarge the diameter (200 to 1,000 μm). Further, even such a large diameter fiber is strong against bending and has excellent characteristics, due to the flexibility of the polymer material. Thus, with POF, a large diameter is possible, whereby injection, connection or branching of optical signals has become very easy.
A graded index (referred to simply as GI) type POF as a result of the research in the past, is a plastic optical fiber made of a polymethyl methacrylate (referred to simply as PMMA), wherein the refractive index gradually decreases from the core center towards the outside in the radial direction of the plastic optical fiber, and the refractive index of the clad is lower than the refractive index at the core center (e.g. JP-A-6-186441, JP-A-6-186442, JP-A-7-27928). With this GI type POF, as shown in FIG. 1, the output waveform after transmission for 100 m has substantially the same shape (the waveform shown by the solid line in FIG. 1) of the input waveform, although it is a waveform having a slight broadening of the arrival time by the influence of the difference in mode. Thus, GI type POF has a small mode dispersion and as compared with conventional SI type (step index type) POF, is epoch-making such that it is possible to improve the communication speed by at least two digids.
Further, taking connection to silica fiber as the trunk line fiber into consideration in an attempt to minimize the attenuation of POF at a wavelength of 1.55 μm at which the attenuation of the silica fiber becomes minimum, POF having hydrogen atoms in the polymer molecule replaced by deuterium atoms heavier than the hydrogen atoms or by fluorine atoms, was prepared (e.g. JP-A-10-268146, JP-A-10-293215). As a result, the absorption wavelength was shifted to the long wavelength side, and the attenuation at the wavelength of 1.55 μm was lowered, and thus it was made possible to obtain POF having a very low attenuation over a wide range of from the visible light region to the near infrared region.
However, as a result of an attempt to improve the transparency by using such a material, a problem of a high order mode dispersion which used to be hidden behind, with the conventional PMMA optical fiber, was surfaced. Specifically, as shown in FIG. 1, in the output waveform, delay in the output which appears to be attributable to an influence of the high order mode, became distinct (the waveform shown by white circles in FIG. 1). Thus, with a fluorine type optical fiber or the like, while it was possible to lower the attenuation by the attempt to improve the transparency, a new problem came up such that the communication speed was remarkably deteriorated.
Therefore, it was strongly desired to develop a technology to minimize the delay time in the high order mode transmission to increase the communication speed, while maintaining the low attenuation.
The present invention is intended to solve the problem in the prior art and to accomplish the following object. Namely, it is an object of the present invention to provide a plastic optical fiber which is low in attenuation in a high order mode and small in mode dispersion.